Multilayer coating systems are well known in the art. For example, in the metal container industry, it is conventional to print or decorate an article after which it is coated with a transparent varnish or film which serves to protect the printing and heighten the aesthetic characteristics of the container. These systems have long been applied separately and baked in ovens to remove volatile solvents and thereby dry the respective layers prior to overcoating with an additional layer which is likewise heated or baked to evaporate the solvent. Evaporation and baking consume large quantities of energy and add pollutants to the atmosphere. For this reason, in an age of conservation of energy consumption and pollution prevention, increasing emphasis is being directed in the industry to energy saving and pollution free techniques such as curing of photopolymerizable coatings, the advantages over prior procedures being readily apparent.
More recently, a broad spectrum of photocurable coatings has been developed wherein light-sensitive catalyst precursors are incorporated in polymerizable materials and act to initiate curing via crosslinking or polymerization upon exposure to irradiation. Many prior art processes disclose radiation curing of various polymers, all of which have advantages and disadvantages peculiar to the particular system. Attempts to utilize the best features of different systems by employing inks from one system in combination with varnishes from another are frequently thwarted by unexpected interactions which result in slow cure. Some inks and varnishes which independently have desirable properties cannot be used in combination with each other. This results in the use of less desirable materials, as for example, volatile materials, which defeats the anti-polluting goal of radiation curing. The limitations of combining inks and varnishes from various systems have contributed to the slow commercial acceptance of radiation curing.
Another consideration that affects the applicability of such radiation-sensitive compositions is the stability of the various components. For example, light-curable inks are well known. It is a common failing of all such inks, however, that they have a limited storage stability. This is particularly pronounced with inks based on epoxides but is also true of acrylics which have been found to gel on storage.